The invention relates to a method and apparatus for inter-operating or interworking a cell-based ATM network with a frame-based ATM network.
Frame-based asynchronous transfer mode (FB-ATM) protocols are currently under development by organizations such as the ATM Forum. FR-ATM differs from the presently well defined cell-relay ATM (CR-ATM) protocols in that the former will employ a variable length frame as the basic unit of data transport whereas the latter employs fixed size cells. An interworking function will be required for seamlessly transporting data between CR-ATM networks and FR-ATM networks.
The CR-ATM protocol is layered or stacked as set forth for instance in the B-ISDN protocol stack described in CCIT Recommendations I.362 and I.363. The protocol includes an ATM adaptation layer (AAL) for adapting the services of the connection orientated ATM Layer to those required by higher layers (commonly referred to as AAL users). Some versions of the AAL operate by receiving user data, such as an Internet Protocol (IP) or Frame Relay (FR) packet, constructing an AAL common part convergence sub-layer protocol data unit (CPCS-PDUxe2x80x94hereinafter referred to as xe2x80x9cAAL PDUxe2x80x9d) based on this data, and segmenting the AAL PDU into plural segmentation and re-assembly protocol data units (SAR PDUs) that form the payload of the transportable fixed-size ATM cell. The AAL also functions in the reverse direction to convert plural SAR PDUs into an AAL PDU. The emerging FB-ATM protocol will likely also be stacked in a manner similar to CR-ATM and include an AAL (hereinafter FB-AAL) equivalent to the CR-ATM AAL.
One intuitive approach to interworking when data originates and terminates in a CR-ATM network and is transferred across an intermediary FB-ATM network is to assemble a series of fixed size ATM cells which collectively correspond to a CR-ATM AAL PDU. This assembly of cells could form the input to the FB-AAL. Conversely, an assembly of FB-ATM frames could form the AAL PDU for segmentation and data transfer across a CR-ATM network. Such an approach is, however, not optimally efficient with respect to the conservation of bandwidth.
Broadly speaking, the invention provides a method of interworking between cell-based and frame-based ATM networks.
According to one aspect of the invention, a method is provided for converting cell-based ATM traffic to frame-based ATM traffic. The method comprises (a) aggregating a group of one or more cells of an input cell stream which corresponds to an AAL, preferably AAL5, PDU; (b) extracting from the cell group a user data payload of the AAL5 PDU, exclusive of any AAL PDU padding; (c) constructing a frame-based ATM adaptation layer protocol data unit (FB-AAL PDU) having no padding using the AAL PDU payload; and (d) segmenting the FB-AAL PDU into one or more frames to generate an output frame stream. The method provides an increase in bandwidth efficiency since the padding overhead required by AAL5 PDUs is eliminated when transferring data to, over or across FB- ATM.
According to another aspect of the invention, a method is provided for converting frame-based ATM traffic to cell-based ATM traffic. The method comprises (a) assembling a group of one or more frames of an input frame stream which corresponds to a FB-AAL PDU; (b)extracting from the frame group a user data payload of the FB-AAL PDU; (c) constructing a CR-ATM AAL PDU using the FB-AAL PDU payload; and (d) segmenting the CR-ATM AAL PDU into one or more cells to generate an output cell stream.
Another more general aspect of the invention provides a method for converting a stream of fixed size packets to a stream of variable sized packets in circumstances where the variable sized packets have a maximum size limit. The method comprises: (a) assembling a group of one or more fixed size packets wherein a last packet of the group includes padding for the purposes of aligning a user data payload for transport over the group of packets; (b) extracting the user data payload from the group of fixed size packets, exclusive of the padding; and (c) segmenting the user data payload into one or more variable sized packets to generate the variable sized packet stream.
Interworking apparatus for effecting the above methods is also disclosed.